Fish Belt Transects (BLT)

0.1 Overview

The fish belt transect (BLT) protocol is the primary method used to quantify reef fish assemblages at UVS sites. Divers swim standardized belt transects, visually identifying and sizing all fishes observed within a defined area. This method yields robust estimates of fish abundance, biomass, and community structure across depth strata and regions (see SOP for full methods).

Fish BLT data are organized across three interlinked tables:

• uvs.blt_stations - Metadata for each station, including depth, diver, transect area, and summary metrics
• uvs.blt_observations - Raw fish counts by species, size class, and transect replicate
• uvs.blt_biomass_by_taxa - Derived biomass by station and taxon, calculated using standard length–weight relationships

All records are linked by ps_station_id, and species are standardized via accepted_aphia_id (foreign key to taxonomy.fish).

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Stations (uvs.blt_stations)

This table contains one record per depth-stratified fish survey conducted at a UVS site. Each row includes core spatial and survey metadata — such as depth, date, divers, and number of transects — along with ecological summaries (e.g., species richness, fish abundance, biomass).

Stations are linked to uvs.sites via ps_site_id, and uniquely identified using a depth-binned ps_station_id. This table provides the context and summary metrics for interpreting observation-level data.

Standard UVS depth strata

To ensure consistency across regions and survey protocols, all UVS stations are binned into standardized depth strata based on fixed thresholds. These depth bins are appended to the ps_site_id to generate the full ps_station_id.

Depth strata and suffixes:

• Supershallow (<= 6 m) → 05m
  
• Shallow (7–14 m) → 10m
  
• Deep (>= 15 m) → 20m

Examples:

• A survey at 5.8 m → FJI_2025_uvs_001_05m
• A survey at 14.7 m → FJI_2025_uvs_001_10m

This scheme ensures consistent stratification across all analyses, even with natural variation in survey depth.

Table 1: Fields in the uvs.blt_stations table

Field	Type	Required	Description
ps_station_id	STRING	true	Unique station ID (ps_site_id_depth), e.g., CHL_2024_uvs_001_20m.
ps_site_id	STRING	true	◂ Foreign key to uvs.sites table.
exp_id	STRING	true	◂ Expedition ID (ISO3_YEAR).
method	STRING	true	◂ Survey method (uvs).
protocol	STRING	true	Survey protocol (fish_blt).
divers	STRING	true	Comma-separated names of divers who conducted the transects.
region	STRING	true	◂ Region name.
subregion	STRING	true	◂ Subregion name.
locality	STRING	false	◂ Optional local feature (e.g., reef, bay, cove).
latitude	FLOAT	true	◂ Latitude of site (decimal degrees, WGS84).
longitude	FLOAT	true	◂ Longitude of site (decimal degrees, WGS84).
date	DATE	true	◂ Survey date (YYYY-MM-DD).
time	TIME	true	◂ Start time of survey (24-hour format).
depth_m	FLOAT	true	Average depth of the station (m).
depth_strata	STRING	true	Depth bin: supershallow, shallow, or deep.
habitat	STRING	true	◂ Habitat type (see controlled vocabulary).
exposure	STRING	true	◂ Exposure type (see controlled vocabulary).
n_transects	INTEGER	true	Number of belt transects completed at the station.
survey_area_m2	FLOAT	true	Total area surveyed across all transects (m²).
total_richness	INTEGER	false	Total number of unique fish taxa recorded.
total_count	INTEGER	false	Total number of individuals recorded.
avg_richness	FLOAT	false	Mean taxa richness per transect.
avg_count	FLOAT	false	Mean number of individuals per transect.
avg_count_m2	FLOAT	false	Mean fish density (individuals/m²).
avg_biomass_gm2	FLOAT	false	Mean biomass (g/m²).
pct_biomass_top_pred	FLOAT	false	Average % of total biomass contributed by top predators.
pct_biomass_sharks	FLOAT	false	Average % of total biomass contributed by sharks.
notes	STRING	false	Optional comments or QA notes.

Observations (uvs.blt_observations)

This table stores raw fish observations recorded during individual belt transects. Each row captures a unique combination of species, estimated total length (cm), and count, representing a single diver observation within a defined transect area.

In addition to raw counts, it includes derived metrics such as biomass (biomass_g) and biomass density (biomass_gm2), calculated using species-specific length–weight parameters from taxonomy.fish. Observations are linked to their parent station via ps_station_id, and taxonomic identity is standardized using accepted_aphia_id.

Standard transect dimensions

• Fish ≥ 20 cm are recorded along a 25 m × 4 m belt (100 m²)
• Fish < 20 cm are recorded along a 25 m × 2 m belt (50 m²)

Table 2: fish.observations Table Schema

Field	Type	Required	Description
obs_id	STRING	true	Unique observation ID (e.g., CHL_2024_blt_AMF_0001).
ps_station_id	STRING	true	Foreign key linking to uvs.blt_stations.
diver	STRING	true	Name of the diver who recorded the observation.
transect	STRING	true	Transect label: A, B, or C; OFF for observations outside standard transects.
station_label	STRING	true	Field-assigned relative stratum label (e.g., shallow, deep).
depth_m	FLOAT	true	Recorded depth (m) at which the transect was conducted.
accepted_name	STRING	true	Scientific name (Genus species) of the observed taxon.
accepted_aphia_id	INTEGER	true	WoRMS AphiaID — foreign key to taxonomy.fish.
family	STRING	true	Family of the observed taxon (from taxonomy.fish).
trophic_group	STRING	true	Trophic group classification (e.g., herbivore/detritivore, top-predator).
terminal_phase	BOOLEAN	false	TRUE if the individual was identified as terminal phase (e.g., parrotfishes).
length_cm	FLOAT	true	Estimated total length (cm) of the individual.
count	INTEGER	true	Number of individuals observed in this size class.
count_m2	FLOAT	false	Fish density (individuals/m²), based on size-specific belt area.
biomass_g	FLOAT	false	Total biomass (g), calculated using length–weight relationship.
biomass_gm2	FLOAT	false	Biomass per square meter (g/m²), standardized by belt area.
notes	STRING	false	Optional comments or QA annotations.

Off-transect observations

Fish observed outside of standard transects (e.g., large pelagics, megafauna) are recorded using transect = "OFF". These are included for completeness but should be treated separately in standardized analyses.

Biomass by taxa and station (uvs.blt_biomass_by_taxa)

This table contains station-level summaries of reef fish abundance and biomass by species, derived from raw BLT transect observations. It serves as the primary analysis-ready table for assessing community structure, trophic composition, and biomass patterns.

Each row represents a unique combination of ps_station_id and species. Metrics are expressed as mean values across all transects at the station, with zeros imputed for transects where the species was not observed. This approach ensures unbiased station-level estimates of:

• count_m2 - average density (individuals/m²)
• biomass_gm2 - average biomass (g/m²)

Species identity is standardized via accepted_aphia_id, with taxonomic and ecological traits (e.g., family, trophic group) joined from taxonomy.fish. These fields support flexible aggregation by trophic or functional guild.

Table 3: Schema for station-level summaries of reef fish biomass and abundance by species

Field	Type	Required	Description
ps_station_id	STRING	true	Unique station ID (ps_site_id_depth), e.g., CHL_2024_uvs_001_20m.
accepted_name	STRING	true	Scientific name of the taxon (Genus species).
accepted_aphia_id	INTEGER	true	Unique AphiaID from WoRMS; foreign key to taxonomy.fish.
family	STRING	true	Family of the taxon (joined from taxonomy.fish).
trophic_group	STRING	true	Trophic group classification (e.g., planktivore, top-predator).
depth_m	FLOAT	true	Mean depth (m) of the station.
depth_strata	STRING	true	Depth bin label: supershallow, shallow, or deep.
region	STRING	true	Region name.
subregion	STRING	true	Subregion (e.g., island, gulf, reef complex).
locality	STRING	false	Optional local feature (e.g., reef, bay, cove).
habitat	STRING	true	Dominant habitat type at the station.
exposure	STRING	true	Wave/wind exposure at the station (windward, leeward, lagoon).
total_count	INTEGER	false	Total number of individuals observed across all transects.
avg_count	FLOAT	false	Mean number of individuals per transect (including zeros).
avg_count_m2	FLOAT	false	Mean density (individuals/m²), with zeros imputed for missing transects.
avg_biomass_gm2	FLOAT	false	Mean biomass (g/m²), with zeros imputed for missing transects.
avg_length_cm	FLOAT	false	Weighted mean total length (cm); weights based on individual counts.
min_length_cm	FLOAT	false	Minimum total length (cm) observed at the station.
max_length_cm	FLOAT	false	Maximum total length (cm) observed at the station.